This invention relates to a process of purifying an oxygen-containing exhaust gas which has been produced by combustion of garbage, industrial waste materials and sewage sludge and which is contaminated with dust, SO.sub.2, HCl, HF, heavy metals, particularly Hg and mercury compounds, and with dioxins and furans.
Because of the different and changing compositions of garbage, industrial waste materials and sewage sludge, the exhaust gases produced by a combustion of these waste materials are contaminated with different amounts of substances which pollute the environment. All impurities must substantially be removed from the combustion exhaust gases before it is permissible to discharge the exhaust gases into the atmosphere because a great number of impurities even in low concentrations produce toxic effects in human beings, animals, and plants. The exhaust gases produced by combustion of waste materials are particularly contaminated with dust, SO.sub.2, HCl, HF, Hg mercury compounds as well as dioxins and furans. In addition to these substances the combustion exhaust gases also contain NO.sub.x, heavy metal compounds, and highly condensed aromatic hydrocarbons as further impurities. Because the waste materials are combusted with oxygen in a stoichiometric excess, the combustion exhaust gases contain 2 to 11% by volume oxygen.
The dust, which is contained in the exhaust gases in an amount of up to 50,000 mg/sm.sup.3 (sm.sup.3 =standard cubic meter), is separated in cyclones, electrostatic precipitators, cloth filters or scrubbers and the exhaust gas to be dedusted may pass through a plurality of such units. At the present time the known dedusting processes even when carried out on a commercial scale deliver a pure gas that contains &lt;5 mg/sm.sup.3 residual dust. The dedusting must be as complete as possible because the dust mainly adsorbs toxic heavy metals, heavy metal compounds and polyhalogenated dibenzodioxins and dibenzofurans.
The SO.sub.2 and HCl are each contained in the exhaust gases in an amount of up to 3000 mg/sm.sup.3 and HF is also present in amounts of up to 100 mg/sm.sup.3. Together with the water vapor contained in the atmosphere these gaseous compounds form acids, which are very often present as aerosols and the gaseous compounds have a toxic activity. For this reason they are substantially removed and the known purifying processes carried out on a commercial scale may result in residual contents of &lt;20 mg/sm.sup.3 SO.sub.2, &lt;5 mg/sm.sup.3 HCl and &lt;1 mg/sm.sup.3 HF. Dry or quasi-dry or wet purifying processes are used to remove SO.sub.2, HCl and HF, and a plurality of processes may be carried out in succession. The reactants used in these processes consist mainly of Ca(OH).sub.2, CaO, CaCO.sub.3, NaOH, and Na.sub.2 CO.sub.3. Spray absorption is a process which has gained special significance and in which an aqueous suspension of Ca(OH).sub.2 reacts with the acid pollutants SO.sub.2, HCl and HF, the water evaporates and a solid reaction product is obtained, which contains also dust and other pollutants. Scrubbing processes are also known, by which the pollutants SO.sub.2, HCl and HF can be removed to a very high degree.
The heavy metals and heavy metal compounds, mainly mercury and mercury compounds, and polyhalogenated dibenzodioxins and dibenzofurans are contained in the combustion exhaust gases in lower concentrations. But these substances have an extremely high toxic activity and, for this reason, must be almost entirely removed from the exhaust gases. This is preferably effected in the prior art by adsorption and/or scrubbing processes. Activated carbon has proved to be a particularly satisfactory adsorbent. The scrubbing processes are carried out under acid or alkaline conditions. Besides, dioxins and furans may be decomposed by catalytic conversion.
To have commercial utility, the known processes of purifying exhaust gases are required to involve only a low capital investment and low operating costs and to produce products which are obtained at a low rate and can be dumped without great difficulties. In order to remove the above-mentioned impurities to the highest possible degree, a plurality of purifying processes are usually combined.
For instance, European Patent Specification 0 253 563 discloses a process in which mercury vapor and/or vapors of deleterious organic compounds and/or nitrogen oxides are removed from a hot exhaust gas stream, which is discharged by a combustion plant and contains also fly ash and acid components--i.e., SO.sub.2 and HCl--. In that known process the exhaust gas to be purified is fed at a temperature from 135.degree. to 400.degree. C. into an atomizing and absorbing chamber, in which an aqueous liquid is atomized, which contains a basic absorbent. In the spray absorber the exhaust gas is cooled at the same time by an evaporation of water to 180.degree. to 90.degree. C. The acid components of the exhaust gas are bound and a particulate material is thus formed, which contains the products of the reactions between the basic absorbent and the acid components of the exhaust gas as well as unreacted absorbent, The particulate reaction product and any fly ash which is present are separated from the exhaust gas in a particle separator downstream of the atomizing and absorbing zone. In the known process, activated carbon powder at a rate of 1 to 800 mg/sm.sup.3 of exhaust gas is introduced into the exhaust gas stream at least at one location, which may be upstream of the atomizing and absorbing chamber or in the atomizing and absorbing chamber or downstream of the atomizing and absorbing chamber but upstream of the particle separator. The activated carbon powder is separated together with the particulate reaction products in the particle separator.
Besides, German Patent Specification 40 12 320 discloses a process of purifying exhaust gases which, in addition to fly ash, contain gaseous pollutants, such as SO.sub.2, HCl and HF, as well as volatile heavy metals, particularly mercury compounds. That known process comprises initially a dry separation of solids and a subsequent two-stage scrubbing with a circulated scrubbing liquid. The first scrubbing stage is operated at a pH from 0 to 3 and the second scrubbing stage at a pH from 6 to 9.
It has been found that more stringent requirements for purity necessitate an improvement of the known purifying processes. It has also been found that in the known purifying processes the pollutant concentrations in the pure gas may increase for short times, particularly as a result of fluctuations of the concentrations of individual pollutants in the combustion exhaust gases, and such increases must be avoided with a view to possible hazards.